Fungi are a diverse group of organisms that can be decomposers, parasites, or mutualistic symbionts with plants and algae. They lack chlorophyll and absorb nutrients from dead or living matter. Fungi reproduce both sexually and asexually through spores. They are classified into groups based on their spore-bearing structures, including club fungi, sac fungi, zygote fungi, and imperfect fungi. Fungi play important ecological roles in decomposition, nutrient cycling, and symbiotic relationships with other organisms.